A conventional rain drop detecting device is disclosed in Japanese Patent Laid-Open Publication No. 4 (1992)-78641. The rain drop detecting device disclosed in this publication includes a detecting unit comprised of a wave sending element and a wave receiving element, each of which is formed with a piezoelectric vibrator. The detecting unit is disposed at the lower portion of the front window of a vehicle. In this way, the driver's view is not obstructed and the detecting device does not detract from the vehicle's attractive appearance.
However, in accordance with this construction, when a large amount of rain water is on the front window of the vehicle and is being wiped by the windshield wiper, the water has a tendency to stick to the wiper. Because the detecting unit is disposed at the lower portion of the front window of the vehicle, the detecting unit has a tendency to detect the water on the wiper, even though the windshield may be clear of water. The detecting unit thus inadvertently detects a large quantity of rain drops on the windshield and so the wiper blade is frequently operated even though rain water on the windshield has been wiped away.
FIG. 4 illustrates the reason for the aforementioned problem. As illustrated in FIG. 4, the detecting device includes vibrators 20, 30, and the area of the windshield over which the ultrasonic wave is generated is represented by the region enclosed within the pair of arched broken lines. As can be seen, the ultrasonic wave is transmitted towards both the upper and lower portions of the windshield because there is no structure that obstructs the transmission of the ultrasonic wave. Thus, the detecting area covers not only the upper region of the windshield, but also the lower region of the windshield where the wiper 25 is positioned during non-operation. This then causes the detecting device to inadvertently detect water on the wiper 25 which is in the stored position. Consequently, water drops are detected even in situations where the windshield is already clear of water drops.
In light of the foregoing, a need exists for a rain drop detecting device that is able to precisely detect the amount of rain drops falling on a vehicle windshield.
It would be desirable to also provide a rain drop detecting device that is low in cost, relatively simple in structure and small in size.